Fixed az driftPattern at transit

3apr10

The fixed azimuth drift routine within cima is
used for various drift surveys. It allows you to start a drift
scan at a particular az and declination. To use the routine:

the observer provides the azimuth and declination of the source.

Cima computes the za and hour angle to start the drift.

To do this, it precesses the declination to current coordinates

It computes the ha,za given the az, dec Current, and the
observatory lattitude.

Note that cima is using the aoLattitude that is .5 arc
seconds less than what the pointing program uses.

Cima also uses the feed coordinates before applying the
model. The final az,za could be slightly different after the model is
applled (this shouldn't be a problem except for hitting any limits).

Cima moves to the az,za at the start of the strip and then waits
for the hour angle to arrive before starting the drift.

Users have been having trouble with the fixed drift routine when
observing declinations close to that of the AO lattitude (18:21:14.2).
The problems are:

Setting az=90 or 270 will cause a divide by zero in cima. The
computation is dividing by the cos(az). We're stuck with this for
awhile.

Close to the aoLat (a degrees or so) cima seems to go to a za
that is different than what the observer expected.

Cima also seems to not be self consistent. For example:

user picks source from the pointing menu

the pointing menu reports an az,za for the current position of
the source.

users starts fixed drift routine and telescope moves to a za
that is lower than what was just reported.

The problem occurs because the declination strip of a source across the
dish may cross the same azimuth twice (at different za's and hour
angles). This occurs when the source declination is close to the
lattitude of ao.